Double confocal scanning microscopes are known from EP 0 491 289. In double confocal scanning microscopy, light from a light source is divided into two partial beams, each partial beam being focused by way of an objective onto a common specimen point. The two objectives are arranged on different sides of the specimen plane that is common to them. At the specimen point or the detection pinhole, the interferometric illumination causes the formation of an interference pattern that, in the context of constructive interference, exhibits a principal maximum and several secondary maxima. Because of the interferometric illumination, it is possible with a double confocal scanning microscope to achieve increased axial resolution as compared to a conventional scanning microscope. The term “axial resolution” will be used hereinafter to refer to the resolution in the direction of the optical axis.
Especially in the context of multi-color fluorescence applications, however, the axial resolution that is experimentally achievable with a generic double confocal scanning microscope is hitherto less than the theoretically possible axial resolution.